The present invention relates to sleep control of a wireless base station that utilizes a passive optical subscriber network for the connection between a base band unit and a remote radio head.
There is a technology for achieving improved throughput per unit area to accommodate rapidly-increasing mobile traffic by installing a large number of wireless base stations (small cells) having smaller cover areas than in the related art. A small cell includes a base band unit (base band unit: BBU) and a remote radio head (RRH).
The BBU performs management control and signal processing for wireless communication. For example, in long term evolution (LTE) service, the BBU modulates an internet protocol (IP) packet received from a higher-tier network into an orthogonal frequency division multiplexing (OFDM) signal and transmits the OFDM signal to the RRH. Moreover, the BBU demodulates an OFDM signal received from the RRH into an IP packet and transmits the IP packet to the higher-tier network.
The RRH amplifies an OFDM signal received from the BBU and transmits the OFDM signal to user equipment (UE) by using an antenna. Moreover, the RRH amplifies an OFDM signal received from the UE and transmits the OFDM signal to the BBU.
As a configuration that connects the BBU and the RRH, there is a central radio access network (C-RAN) configuration. In a C-RAN configuration, a plurality of BBUs are integrated in a single device, and the BBUs are respectively connected to RRHs in a one-to-one fashion by using optical fibers. Accordingly, in a C-RAN configuration, a large number of small cells can be efficiently installed.
However, in a C-RAN configuration, the number of required optical fibers is proportional to the number of installed small cells. Therefore, the cost for maintaining and setting the optical fibers undesirably increases as the number of installed small cells increases.
In view of such circumstances, the use of a passive optical network (PON) as the network between the BBU and the RRH, namely, mobile front-haul (MFH), has been proposed by, for example, Yazawa et al., “Low-latency transmission technique for mobile fronthaul based on TDM-PON system”, Proceedings of the Institute of Electronics, Information, and Communications Engineers (IEICE) Society Conference, Vol. Com. 2, B-8-38, September 2013 (referred to as “Non Patent Literature 1” hereinafter).
A PON has a single optical line terminal (OLT) provided within a station, a plurality of optical network units (ONUs) individually provided in subscribers' homes, and an optical coupler. The OLT, the ONUs, and the optical coupler are connected to one another by a so-called star-type optical fiber network. In a star-type optical fiber network, a single optical fiber is used for the connection between the OLT and the optical coupler. This single optical fiber is split by the optical coupler so as to be shared by the plurality of ONUs.
In a PON, a signal (sometimes referred to as “uplink optical signal” hereinafter) to be transmitted from each ONU to the OLT is multiplexed at the optical coupler and is subsequently transmitted to the OLT. On the other hand, a signal (sometimes referred to as “downlink optical signal” hereinafter) to be transmitted from the OLT to each ONU is demultiplexed at the optical coupler and is subsequently transmitted to each ONU. In order to prevent interference between the uplink and downlink optical signals, different wavelengths are allocated to the uplink and downlink optical signals.
In a PON, various kinds of multiplexing techniques are used. Examples of multiplexing techniques used in a PON include time division multiplexing (TDM) in which a short segment on the time axis is allocated to each subscriber, wavelength division multiplexing (WDM) in which different wavelengths are allocated to the respective subscribers, and code division multiplexing (CDM) in which different codes are allocated to the respective subscribers. Non Patent Literature 1 described above proposes the use of a PON that utilizes TDM (TDM-PON) as MFH.
In the case where a PON is used as MFH, the BBU is connected to the OLT. Moreover, the RRHs are respectively connected to the ONUs in a one-to-one fashion. In a PON, the users of the respective RRHs can share the star-type optical fiber network, so that the number of required optical fibers can be reduced, as compared with a C-RAN configuration.
Nowadays, for example, the installation of star-type optical fiber networks for fiber-to-the-home (FTTH) service has been extensively completed. Therefore, by utilizing unused star-type optical fiber networks, the cost for installing new optical fibers can be reduced.
Furthermore, by accommodating the BBUs in the same station as the OLT installed for an expedited service in the related art, such as FTTH, advantages, such as a space saving configuration and reduced power consumption owing to improved bandwidth utilization efficiency, can be expected.
As a sleep technology in a PON, for example, Fumio Daido et al., “Development of Communication LSI for 10G-EPON”, SEI Technical Review, No. 180, January 2012 (referred to as “Non Patent Literature 2” hereinafter) propose an ONU power saving method that is standardized based on Institute of Electrical and Electronics Engineers (IEEE) P1904.1 Standard for Service Interoperability in Ethernet Passive Optical Networks (SIEPON). In this method, the OLT first transmits a SLEEP-ALLOW signal to an ONU that is not performing communication. The ONU receiving the SLEEP-ALLOW signal transmits a SLEEP-ACK signal to the OLT. Then, the ONU that has transmitted the SLEEP-ACK signal causes a transmitter and a receiver thereof to sleep. The state of the ONU whose transmitter and receiver are sleeping is referred to as a sleep state.
The ONU in the sleep state cancels the sleep mode of the transmitter and the receiver at every specific time so as to switch to an active state. If there is no communication occurring in the ONU, the ONU switches from the active state to the sleep state again. By periodically repeating the sleep state and the active state in this manner, the ONU can maintain its connection with the OLT (i.e., establish a so-called PON link). Furthermore, when communication occurs in the ONU in the sleep state, the communication can be commenced with low delay.
Accordingly, by setting an ONU not performing communication in a sleep state, power consumption in the PON can be reduced.